Abstract Cachexia is a condition characterized by progressive skeletal muscle and body weight loss and affects up to 80% of cancer patients. Since this loss of muscle mass contributes to weakness, reduced tolerance to conventional treatments, and increased mortality, understanding the mechanisms that drive muscle wasting is critical to the development of treatments to improve quality of life and enhance survival of cancer patients. Aim 3 of the parent RO1, aimed to identify a gene signature that associates with muscle wasting in cancer patients and tumor-bearing mice. To do this, we conducted genome-wide transcriptomic analyses of skeletal muscle obtained from patients with pancreatic ductal adenocarcinoma (PDAC), who have one of the highest rates of cachexia, and in multiple models of cancer cachexia. Included among the models studied was the patient derived xenograft (PDX) model, in which tumors resected from PDAC patients are directly sutured to the mouse pancreas, where they propagate and induce cachexia. From these studies we identified genes commonly changed (?1.5 ? fold change ? 1.5, q ? 0.01) in skeletal muscle from cachectic cancer patients, and in 9 different cohorts of mice in response to tumor burden, including 5 different PDAC-PDX cohorts created from 5 PDAC patients, the C26 model, and mice injected orthotopically with various pancreatic cancer cell lines, including human L3.6pl and Panc-1 cells and mouse KPC cells. One gene commonly changed across these 9 mouse models and in patients was Trim63 (MuRF1), a well-established regulator of skeletal muscle wasting in various physiological and pathophysiological conditions. Despite this, the role of MuRF1 in tumor-induced muscle atrophy is not known. We therefore conducted a preliminary experiment to test this role using MuRF1 knockout (MuRF1-/-) mice, hypothesizing that deletion of MuRF1 would attenuate tumor-induced atrophy. To our surprise, our findings were far more striking. Knockout of MuRF1, which is muscle-specific, completely prevented the cancer-induced loss of muscle mass and fat mass, significantly delayed tumor growth (by nearly the rate of growth in WT mice) and more than doubled survival. Importantly, the complete preservation of muscle and fat mass in MuRF1-/- mice was not confounded by differences in tumor size, as these tissue measurements were made at time points in which KPC tumors in MuRF1-/- mice were equal in size to that reached in WT mice. These striking preliminary findings provide an incredible foundation on which we propose to build through this STAR Program award. Our proposed studies will a) identify the proteins which are ubiquitinated and degraded in skeletal muscle in response to tumor burden, that require MuRF1 and, b) determine the extent to which loss of MuRF1 in tumor bearing mice: i) alters the metabolome in muscle, fat, tumor and serum; ii) alters the tumor- associated dysregulation of circulating growth factors/cytokines in the circulation which associate with the cachectic phenotype and iii) attenuates muscle weakness. Findings from these studies will provide a wealth of new data to facilitate the continued expansion of our research program.